Alkaline storage battery pole plate and production method for alkaline storage battery pole plate and alkaline storage battery

ABSTRACT

A battery used by rolling a pole plate, formed by filling an active material, around a substrate using a band-shaped metal porous body having three-dimensionally linked spaces is inferior in flexibility and likely to short-circuit. According to this invention, grooves are formed in active material-filled substrate filled with the above active material, and then the grooved substrate is pressed flat to form groove active material layers, whereby cracks are made in the bottoms of the grooves preferentially, and cracks are pressed by the groove active material layers and blocked to prevent the flow-out of swelled projections of the cracks and the active material. Accordingly, a higher-capacity and higher-reliability battery is provided.

Technical Field of the Invention

The present invention relates to electrodes for alkaline storagebatteries, a method for manufacturing electrodes for alkaline storagebatteries, and improvement of alkaline storage batteries.

Background of the Technology

Alkaline storage batteries as represented by nickel-hydrogen batteriesand nickel-cadmium batteries, etc., are small, light weight and provideshigh output densities. In recent years, their field of practicalapplications is not limited to small devices such as personal computersand mobile phones but is extending to the area of large size powersources such as for electric vehicles and hybrid electric vehicles. Inassociation with the expanding applications of these batteries,increasingly higher capacity and reliability are now being required.

In alkaline storage batteries, a prismatic structure in which a batteryis configured by assembling plate-form positive electrodes and negativeelectrodes with separators interposed and housing in a prismaticcontainer, and a cylindrical structure in which a battery is configuredby spirally winding a rectangular positive electrode and a negativeelectrode with a separator interposed thus configuring an electrodegroup and housing it in a cylindrical container, are generally adopted.

On the other hand, in recent years, three-dimensional porous metalsubstrates (hereinafter porous substrates) such as foam metal of nickelor non-woven metal fabric having three-dimensionally connected spacewith as high a porosity as exceeding 90% have been developed, and a newmethod for manufacturing electrodes by directly filling paste of activematerial particles into these substrates has been developed, and theseelectrodes are now in wide use in the above-mentioned prismatic andcylindrical batteries as the electrodes suited for higher capacity.However, it has become clear that there exist reliability related issuesarising from the manufacturing method and structure of the electrodesemploying the above-mentioned three-dimensional porous substrate. It isan object of the present invention to improve the electrodes from thestandpoint of the structure and the manufacturing method, and toestablish reliability of alkaline storage batteries provided withelectrodes that employ the above-mentioned three-dimensional poroussubstrate.

As an active material paste to be filled into a porous substrate, activematerial paste for the positive electrode of an alkaline storage batterythat contains nickel hydroxide as the main constituent added with amaterial required for electrode reaction such as cobalt metal, nickelmetal, and carbon powder, and active material paste for the negativeelectrode that contains hydrogen absorbing alloy or cadmium hydroxide asthe main constituent added with a material such as carbon powder andnickel powder required for electrode reaction, and a binder such ascarboxymethyl cellulose and the like are in use.

Electrodes using three-dimensional porous bodies filled with theabove-mentioned active materials have been generally used as a positiveelectrode or a negative electrode of a battery after being pressed intoa predetermined thickness after the paste was filled.

It is common that a dense surface layer has been formed on the surfaceof an electrode pressed as above. Such a dense surface layer hamperspenetration of liquid electrolyte into the inside causing dispersion inthe quantity of injected liquid electrolyte from battery to battery thussuffering from dispersion in the characteristics.

On the other hand, the above-mentioned three-dimensional porous body isa structural material with an originally low degree of freedom ofdeformation against bending force. When an electrode fabricated byfilling an active material into such a structural material and furtherpressing the degree of freedom against bending force is further limited.When trying to configure an electrode group by forcefully winding theelectrode, disorderly cracks, may occur on the outside of the electrodebeing wound, or a squarishly wound electrode group with poor roundnessis formed, thereby causing failure when inserting into a cylindricalmetal container. Furthermore, at the above-mentioned cracks, burrs fromthe damaged portion may project from the surface or active materialparticles may flow out from the damaged portion, thus penetrating theseparator and causing short-circuits of various sizes, further causinginitial or time-varying voltage failures or short-circuit failures.

As a prior art for improving the above issues, a technique has beenproposed as disclosed in Japanese Laid-Open Patent Application No. Sho60-133655, in which V-shaped grooves are formed on both sides of anelectrode and winding it with the direction of the grooves in parallelto the axis of winding. Furthermore, in Japanese Laid-Open PatentApplication No. Hei 5-41211, disclosure has been made on grooves havinga trapezoidal or semi-elliptical cross section instead of grooves havinga V-shaped cross section with which a porous metal body is easy tofracture. However, formation of grooves on both sides is disadvantageousin increasing the quantity of active material to be filled.

Accordingly, as a method for manufacturing by uniformly forming groovesby making the distribution of filled active material uniform, one inwhich active material paste is filled from one side of athree-dimensional porous metal body toward the opposite side has beenproposed in Japanese Laid-Open Patent Application No. Hei 9-106814.Also, a technique is disclosed in which a layer filled with ahigh-density active material is formed on the side of theabove-mentioned filled surface by filling the active material in amanner such that it hardly passes to the opposite side, and a layerfilled with a low-density active material or a non-filled layer isformed on the opposite side, and grooves are formed on the surface ofthe low-density filling side. Furthermore, in the same JapaneseLaid-Open Patent Application No. Hei 9-106814, a description is made onexamples of a method of manufacturing in which grooves or rifts areformed on one side of a three-dimensional porous material body prior tofilling paste and a method of manufacturing an electrode in which activematerial paste is filled from the side opposite to the side on which thegrooves or rifts are provided, and further, on a structure in whichelectrodes are wound in a manner such that the above-mentioned groovesface outward.

On the other hand, in Japanese Laid-Open Patent Application No. Hei9-27342, an electrode comprising a high-density active material-filledlayer and a low-density active material-filled layer similar to the onedisclosed in Japanese Laid-Open Patent Application No. Hei 9-106814 isdisclosed, and both of a structure made by winding with the low-densityactive material-filled layer facing inward and a structure made bywinding with the layer facing outward are disclosed.

In addition, as an example, a description is made on a method ofmanufacturing in which grooves or rifts are provided on athree-dimensional porous body prior to the step of filling an activematerial and active material paste is filled from the side opposite tothe side where the grooves or rifts have been provided, and a method ofmanufacturing in which an active material is filled conversely from theside where the grooves or rifts have been provided.

In either case, electrodes having grooves as described above showedimproved flexibility in a configuration in which the electrodes had beenwound with the side having grooves facing outward due to freedom ofextension of the surface as given by cracks occurring preferentiallyinside the grooves, and tended to cause fewer voltage failures.

Nevertheless, it has become clear that many voltage failure cases stilloccur. From the analysis of the causes, it was found that, in the aboveexisting groove forming configuration, burrs of cracks occurring insidethe grooves either bulge and project out by the winding force or activematerial particles from the cracks flow out to the outer periphery ofthe electrodes passing through the grooves, thus causing new shortcircuits.

To summarize, it was found that inside of the grooves formed on thesurface of a substrate filled with an active material was an emptyspace, and was unprotected against bulging and projection of burrs dueto cracks occurring inside the grooves or against flowing out of activematerial particles from the cracks, thus creating causes for reductionin reliability.

Also, in an electrode fabricated by forming in advance grooves or riftson one side of a three-dimensional porous body and filling an activematerial from the surface provided with the above grooves or rifts, eventhough the active material is filled in the grooves or rifts, the activematerial particles filled in the grooves or rifts are fluidized whendampened by a liquid electrolyte and easily flow out from the grooves asthe active material existing there is simply an aggregate of activematerial particles as filled. It was found that an aggregate of theactive material fluidized as above did not have any control over bulgingand projecting of burrs at cracks and flowing out of the activematerial, or it rather caused new short circuits.

That is, in order to enhance reliability of alkaline storage batteriesthat employ electrodes using a three-dimensional porous metal body whileattaining a higher capacity, new issues have become clear, especially incylindrical batteries, such as development of an appropriate electrodestructure for controlling bulging and projecting out at cracks thatoccur inside the grooves and flowing out of active material particles,and a method of fabrication appropriate for manufacturing the electrodestructure while improving penetrability of liquid electrolyte intoelectrodes.

DISCLOSURE OF THE INVENTION

In order to address the above issues, the present invention discloses anelectrode for an alkaline storage battery in which grooves are formed onthe surface of an active material-filled substrate comprising a porousmetal substrate having three-dimensionally connected space and an activematerial filled in it and, by pressing the active material filledsubstrate, a coarse in-groove active material layer with a low activematerial filling density in the grooves and a dense surface layer with ahigh active material filling density are alternately forming a nearlysmooth surface.

The present invention also discloses, as a method of manufacturing foreffectively forming the above structure, a method of manufacturing anelectrode for an alkaline storage battery that comprises steps offilling an active material in which a filled substrate is formed byfilling active material paste into a porous metal substrate havingthree-dimensionally connected space, forming grooves on one side of thefilled substrate, and pressing the groove-formed electrode to obtain anearly smooth surface.

Furthermore, an alkaline storage battery having an electrode group madeby winding an electrode obtained by the above method with the sidehaving the grooves facing outward and with the direction of the groovesand the axis of winding in parallel to each other is disclosed.

The electrode of the present invention has on its surface a coarsein-groove active material layer, and the layer provides a channel forpenetration of a liquid electrolyte thereby improving penetrability ofthe electrode for the liquid electrolyte. Also, although the grooveshave apparently disappeared as the surface had been smoothed, an effectof improving flexibility is obtained in a structure in which a pluralityof grooves are formed in parallel. Furthermore, as cracks occurringinside the grooves are pressed by the in-groove active material layerformed by pressing, bulging and projection of burrs and flowing out ofthe active material can be controlled.

Exemplary embodiments of the present invention will be described in thefollowing.

The electrode for an alkaline storage battery of the present inventionhas a plurality of parallel grooves on one side of an activematerial-filled substrate comprising a porous metal substrate havingthree-dimensionally connected space and an active material filled in it.In the grooves, a nearly smooth surface is formed by a coarse in-grooveactive material layer and a dense surface layer with a high activematerial filling density formed by pressing.

In the present invention, a high capacity, high flexibility electrodecan be obtained by forming an active material layer in the grooves thatwere empty in the prior arts disclosed in Japanese Laid-Open PatentApplications No. Sho 60-133655 and No. Hei 5-41211. Also, the electrodefor an alkaline storage battery of the present invention ischaracterized by having a coarse and a dense surfaces formed byalternately and regularly repeating in parallel a dense surface layerand a coarse surface formed on the in-groove active material layer, andis superior in absorbing liquid electrolyte. A structure available withthe present invention cannot be expected from making grooves or rifts ina porous metal substrate in advance as disclosed in Japanese Laid-OpenPatent Applications No. Hei 9-106814 and No. Hei 9-27342.

Furthermore, as the electrode for an alkaline storage battery of thepresent invention does not have fractured portion on the skeleton: of aporous metal body that is in contact with the in-groove active materiallayer, occurrence of short circuits due to projection of burrs iscontrolled when wound as an electrode of an alkaline storage batterywhile at the same time an electrical conduction channel is maintained,and when used as an electrode of a battery, internal resistancedecreases compared to the prior arts and large-current dischargecharacteristic is improved.

In an example of the manufacturing method to obtain an electrode for analkaline storage battery of the present invention, the steps of fillingan active material in which an active material-filled substrate isformed by filling an active material into a porous metal substratehaving three-dimensionally connected space, forming grooves on one sideof the active material-filled substrate, and nearly smoothly pressingthe electrode formed with the grooves to a predetermined thickness aresuccessively performed.

An electrode for an alkaline storage battery of the present inventionwith a uniform filled quantity of active material is obtained byfilling, in the step of filling an active material, active materialpaste from one side of a porous metal body in a manner such that it willnot penetrate to the opposite side, and forming grooves, in the step offorming grooves, on the side the active material paste is filled.

When configuring a cylindrical alkaline storage battery by using anelectrode for alkaline storage battery of the present invention in atleast one of the positive electrode and the negative electrode and anelectrode group wound with a separator interposed, winding is performedwith the side having the in groove active material layer facing outwardand the direction of the grooves and the axis of winding in parallel.

During this process, cracks are preferentially formed in the woundelectrode starting at the line of intersection of the bottom and thewalls of the plurality of parallel grooves. In the present invention,the cracks are not formed on the outer periphery of the electrode groupexcept on the bottom of the grooves, and the depth of the cracks isshallower compared to prior arts thus lowering internal resistance ofthe battery and improving large-current discharge performance.

Also, the cracks are pressed by the in-groove active material layer,thereby controlling flowing out of the active material and projection ofburrs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating the steps of obtaining anelectrode of the present invention.

FIGS. 2A-C are drawings to illustrate change of the cross-sectionalconfiguration of an active material-filled substrate in accordance withthe method of manufacturing of the present invention.

FIGS. 3A-C are cross-sectional views of a groove-formed substrate in anexemplary embodiment of the shape of a groove.

FIG. 4 is a disassembled perspective view of a cylindrical alkalinestorage battery of the present invention.

FIG. 5 is a plane cross-sectional view of an electrode group of acylindrical alkaline storage battery according to the present invention.

FIG. 6 is a graph comparing speed of penetration of alkaline liquidelectrolyte in an electrode, of the present invention with that of aprior art electrode.

FIG. 7 is a graph comparing number of occurrence of voltage failuresbetween an electrode of the present invention and that of a prior artused in a cylindrical alkaline storage battery.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to drawings and examples, a description of the preferredembodiments of the present invention will be given below.

FIG. 1 is an example of a preferred device for fabricating an electrodefor an alkaline storage battery of the present invention. By passingthrough a grooved roller 2 having ring-shaped ridges and smoothingroller 3, a substrate 1 filled with an active material is formed into agroove-formed substrate 5 having grooves 4 on one side.

Subsequently, by pressing the groove-formed substrate 5 with a pair ofsmooth pressing rollers 6, the active material oozes out in the grooves4 thus forming an in-groove active material layer 7 and providing anelectrode substrate 8 of the present invention.

Normally, the electrode substrate 8 is cut in the directionperpendicular to the direction of the grooves to obtain an electrode 9of the present invention.

FIG. 2 illustrates the process of change, in terms of thecross-sectional configuration perpendicular to the direction of thegrooves, of the active material-filled substrate in accordance with themanufacturing process of the present invention. In FIG. 2, (A) is theactive material-filled substrate, (B) is the, groove-formed substrate,and (C) is the electrode substrate.

In the present invention, there are no restrictions on the thickness,porosity, pore diameter, shape of pore, etc., of the three-dimensionalporous body, and existing three-dimensional porous bodies can be used.Depending on the purpose, generally used porous bodies with 1 mm to 3 mmthickness and 90% to 95% porosity are used.

For the fabrication of an active material-filled substrate, a method ofusing a porous body previously cut into a strip or flat plate and amethod of using a continuous belt of porous body and cutting it at anarbitrary later step to the shape of an electrode are used. The latteris superior in mass producibility.

In the present invention, there is no particular limitation on themethod of filling an active material, and any method cane be employedinsofar as uniform filling can be made. For example, existing methodsgenerally employed for filling an active material can be employed suchas to fill an active material by passing a porous substrate through abath of active material paste or to fill an active material by pressingfrom one or both sides by means of a fixed-volume ejection nozzle or adoctor knife.

As the active material paste, positive active material paste havingnickel hydroxide as the main constituent and blended with necessaryadditives and a binder, and negative active material having cadmiumhydroxide or metal hydroxide as the main constituent and blended withnecessary additives and a binder are used.

Next, a description will be made on the step of forming grooves. By thisstep of forming grooves, the cross-sectional configuration changes fromFIG. 2(A) to (B).

In forming grooves 4 on a belt-shaped active material-filled substrate1, it is convenient to use a grooved roller 2 provided with a pluralityof annular ridges on the periphery of the roller and a smoothing roller3. On one side of the substrate 5 having belt-like grooves obtained bypassing the rollers, a plurality of continuous grooves 4 are formed overthe width in the direction of the belt-like substrate.

The first purpose of forming the grooves 4 is to apply a stress to thebottom 10 of the grooves thereby causing cracks to take placepreferentially in the bottom 10 of the grooves when winding. The secondpurpose is to secure a space for forming a preferable in-groove activematerial layer 7 in the grooves with the active material caused to oozefrom the skeleton of the porous body.

In the electrode of the present invention, the grade of the in-grooveactive material layer is affected by the configuration of thegroove-formed substrate. For example, it is affected by the ratio of thedepth of the grooves to the thickness of the groove-formed substrate,the state of damage of the metal skeleton of the porous substrate in thevicinity of the grooves of the groove-formed substrate, shape of thegrooves, etc.

When the depth ratio of the grooves is too small, the effect itself ofimproving windability expected from the present invention is diminished.Also, the active material layer inside a groove becomes insufficientthus being unable to form an in-groove active material layer.Conversely, when the depth ratio is too large, skeleton of, the porousmetal body becomes fragile, the active material inside a groove becomesexcessive, and the densities of the in-groove active material layer andits surface layer increase.

Taking the above tendency into account, it is preferable to make theratio of the depth of a groove to the thickness t2 of the groove-formedsubstrate 5 in the range from 20% to 50%.

If an area exists nearby the walls of a groove where the skeleton of theporous metal body is damaged, when such a substrate is employed as abattery electrode of the present invention, the damaged area causesshort circuits and at the same time an excessive amount of activematerial flows into the groove thus making the densities of thein-groove active material layer and its surface layer too large.Consequently, it is important in this invention that the porous metalbody of a groove-formed substrate does not have a damaged area in theskeleton of the porous metal body.

In the present invention, several preferable embodiments are made clearin which damage of the skeleton of a porous metal body can be preventedand an appropriate stress can be applied to the bottom 10 of a groove.

One of them is an embodiment in which a flat surface is made on thebottom 10 of a groove in parallel to the surface of the substrate. Inthis case, it is preferable to chamfer the intersections of the flatsurface and walls of the groove to the shape of an arc, for example.

A configuration of a groove as constructed by walls as designed based onthe shape formed by the outer peripheries of the two arcs as illustratedin FIG. 3(A), (B), (C) and a flat surface of the bottom 10 of the grooveis found to be a preferable embodiment in that it applies a strongstress to the bottom 10 of the groove as well as prevents breakage ofthe skeleton of the porous metal body, and furthermore it tends to causecracks preferentially on the bottom 10 of the groove when it comes tothe step of being pressed.

When the cross section of the walls of the groove is formed with arcscrossing each other, both embodiment of FIG. 3(A) and embodiments (B),(C) formed by separated arcs are preferable embodiments, andconfigurations based on these configurations provide similar effects.

Defining the angle formed by the tangents of the two arcs at theintersection where they cross the flat area 10 on the bottom of thegroove to be an angle of invasion, G, as illustrated in FIG. 3, it ispreferable that G be in the range from 45° to 100°. When the angle ofinvasion, G, is too large, it becomes difficult to give a strong stressto the skeleton of the bottom of the groove and to secure necessarydepth of the groove. Conversely, when it is small, there is possibilityof occurrence of damage nearby the front end of the groove when formingthe groove and possibility of occurrence of a trouble in which thegrooves 4 bite the grooved roller 2 and will not be detached.

Next, a description will be given on the step of forming an in-grooveactive material layer.

This step is one in which a groove-formed substrate 5 is pressed in amanner such that the entire groove-formed substrate including thegrooves forms a nearly smooth surface. It is simple to perform smoothingpressing by means of a pressing roller 6 having a smooth surface. Duringthis step, the cross-sectional configuration changes from FIG. 2(B) to(C).

That is, an in-groove active material layer 7 is formed in the groove 4that was a cavity at the stage of being a groove-formed substrate 5,thus providing an electrode substrate 8 of the present invention.

In this step, the state of dampness of the groove-formed substrate 5 tobe pressed is arbitrary. However, when pressed under a damp condition,flowing out of the active material increases making control difficult.Furthermore, the active material sticks to the pressing device possiblymaking the pressed surface non-uniform. Accordingly, it is preferable topress under a dry condition.

Also, at this step, it is better to perform pressing using a pressingroller having an axis perpendicular to the direction of the grooves.Pressing using a pressing roller having an axis parallel to thedirection of the grooves, that is, pressing in a direction perpendicularto the direction of the grooves causes elongation or warping of thegroove-formed substrate, and will further result in the reduction ofstrength of the porous body substrate.

Different from a groove-formed substrate fabricated by a prior art, thesurface of the electrode substrate 8 of the present invention is anearly smooth coarse and dense surface due to an in-groove activematerial layer 7 formed by pressing the grooves of a groove-formedsubstrate and a dense surface layer 11 formed by pressing the areasother than the grooves. As a coarse surface and a dense surfacegenerally have different reflectivity, a stripe pattern is observed onthe surface of an electrode substrate 8 having a plurality of grooves.The observed color changes depending on the color and composition of theactive material. For example, with a nickel positive electrode using agreen or blackish brown active material, a stripe pattern formed by ablackish brown surface and a surface with whitish streaks correspondingto the grooves is observed suggesting that the surface is not uniform.

Subsequently, as illustrated in FIG. 1, by cutting a predeterminedlength equal to the width of an electrode in the direction of the lengthof the belt-like electrode 8, an electrode 9 of the present invention isobtained on which a plurality of parallel grooves have been formedperpendicular to the direction of the length of the electrode.

FIG. 4 illustrates the state in which the electrode 9 is used as apositive electrode of a cylindrical battery.

In FIG. 4, an electrode group 15 is constructed by spirally winding apositive electrode. 12 and a negative electrode 13 with a separator 14interposed. The electrode group 15 is housed in a cylindrical metal case16, the bottom of a positive terminal 17 is electrically connected tothe positive electrode 12, and the metal case 16 is electricallyconnected to the negative electrode 13. Furthermore, after pouring analkaline liquid electrolyte, the electrode group is sealed with asealing plate 18 having the convex positive terminal 17 and a safetyvent.

FIG. 5 is a cross-sectional view of the electrode group 15 of thecylindrical battery as cut by a plane parallel to the bottom of thebattery. As illustrated in FIG. 5, the positive electrode 12 is woundwith the side having the in-groove active material layer 7 facingoutward and with the axis of winding in parallel to the direction of thegrooves.

By applying an electrode 9 as obtained by the present invention to acylindrical battery as has been described above, various useful effectscan be obtained as below.

(1) Different from previous grooved electrodes, grooves of the electrode9 of the present invention are not empty space; although the fillingdensity is low, they have an active material layer 7 providing a highfilling density as an electrode as a whole.

(2) By forming the surface of the in-groove active material layer 7nearly flat with the dense surface layer 11, it is possible to configurea pressing layer for effectively, pressing the in-groove active materiallayer 7 into the grooves with a separator 14, which is disposed on thesurface when winding the electrode 9, working as the pressing surface.

(3) By pressing until the coarse and dense surfaces become nearly flat,it is possible to integrate the in-groove active material layer 7 withthe electrode 9 and keep the active material in the in-groove activematerial layer 7 on the electrode until at least it is wound.

(4) By making the in-groove active material layer 7 and the surfacelayer 11 as pressed molds, it is possible to prevent fluidization andflowing out of the active material from causing new short circuits whena liquid electrolyte is poured into the electrode group 15, and at thesame time solidify the in-groove active material layer 7 and effectivelytransmit pressing force to the inside of the grooves.

(5) By forming the in-groove active material layer and its surfacecoarse, that portion provides a channel for penetration of the liquidelectrolyte while providing flexibility to the in-groove active materiallayer and freedom of elongation to the surface thus greatly improvingwindability of the electrode.

(6) In the event a force causing a crack on the electrode 9 is applied,a crack 19 is urged to occur preferentially on the bottom 10 of a groovethereby preventing cracks from occurring irregularly over the entiresurface of the electrodes 9.

When the electrode 9 of the present invention is wound as illustrated inFIG. 5, a crack 19 is urged to occur preferentially on the line ofintersection of the bottom 10 of the groove and the walls, and thein-groove active material layer 7 is elongated to allow smooth winding.The phenomenon that the crack 19 occurs preferentially on the bottom 10of the groove deformed from the original groove 4 even after pressingand smoothing a once-formed groove 4 in this way until it apparentlydisappears thus improving windability is a new discovery which has neverbeen predicted.

(7) Also, in the present invention, grooves are formed in a manner suchthat the skeleton of a metal porous body is not damaged in the step ofgroove forming, and at the same time power receiving ability of theporous substrate is maintained as the length of cracks 19 that occurafter winding is short. As a result, internal resistance of the batteryis lowered and large-current discharge performance is improved comparedto the prior art.

(8) In a wound electrode group 15, the surface of the in-groove activematerial layer 7 is pressed by an adjacent separator 14, its force ofpressing is transmitted to the bottom of the grooves, thus pressing andclosing the cracks 19 and preventing bulging and projecting out of thedamaged portion or flowing out of the active material.

Next, in order to verify the effect on reliability of an electrode 9 ofthe present invention as applied to an alkaline storage battery, apositive electrode for an alkaline storage battery in accordance withthe present invention was fabricated, and a cylindrical alkaline storagebattery using it was fabricated. At the same time, an electrode with aprior art configuration was fabricated and the characteristic relatingto reliability of the electrode and battery of the present invention wascompared.

EXAMPLE 1

Active material paste was prepared by adding to 100 parts by weight ofnickel hydroxide 0.2 parts by weight of carboxyl methyl cellulose as abinder and water so that it equals 25% by weight of the total weight ofthe paste.

A 1.7 mm-thick belt-like active material-filled substrate 1 wasfabricated by using as the support a belt-like foam metal porous body115 mm in width, 1.7 mm in thickness, with a porosity in the range 92%to 95%, and having three-dimensionally connected space, and filling theactive material paste from both sides of the porous body.

Subsequently, by using a grooved roller 2 provided with a plurality ofannular ridges on the outer periphery of the roller and a smooth roller3, 0.35 mm deep grooves 4 that continue in the direction of the progressof the substrate are formed on one side of the belt-like activematerial-filled substrate 1 at a pitch of 1.5 mm in the direction of thewidth of the belt-like substrate. The thickness of the groove-formedsubstrate 5 became 1.0 mm. The shape of the grooves was chosen to be asillustrated by FIG. 3(A). Next, by using a smoothing roller 6, thegroove-formed substrate 5 was pressed to a thickness of 0.8 mm to oozeout the active material into the grooves while the in-groove activematerial layer 7 was press-formed to make a nearly smooth coarse anddense surface.

From this belt-like substrate, an electrode 9 for an alkaline storagebattery, 61 mm in width and 110 mm in length, was fabricaed. Theelectrode is referred as electrode P.

Separately, a negative electrode 13, 61 mm in width, 145 mm in length,and 0.4 mm in thickness, was fabricated by coating on a punched metalpaste having hydrogen absorbing alloy powder as the main component.Using the electrode P as a positive electrode 12, an electrode group 15was constructed by winding it together with the negative electrode 13with a separator 14 interposed. In doing this, the positive electrode 12was wound in a manner such that the side provided with grooves faceoutward. An HR17/67 size cylindrical alkaline storage battery, asillustrated in FIG. 4, having a nominal capacity of 3800 mAh wasfabricated by housing the electrode group 15 in a cylindrical metal case16, connecting terminals, pouring a liquid electrolyte, and sealing witha seal plate 18. This battery is referred to as battery P.

COMPARATIVE EXAMPLE 1

Setting the thickness of a smooth pressed substrate at 0.8 mm, anelectrode was fabricated under the same condition as the example of thepresent invention with the exception of cutting electrode configurationwithout going through the groove forming step and the in-groove activematerial layer forming step. The electrode obtained is referred to aselectrode Q. Also, a cylindrical alkaline storage battery was fabricatedwith the same structure as the example of the present invention with theexception of using the electrode Q as the positive electrode 12. Thisbattery is referred to as battery Q.

COMPARATIVE EXAMPLE 2

Setting the thickness of a groove-forming substrate at 0.8 mm, anelectrode was fabricated under the same condition as the example of thepresent invention with the exception of doing away with the in-grooveactive material layer forming step and making the depth of the groovesnearly the same as that of the grooves of the positive electrode 12 ofthe present invention. This electrode is referred to as electrode R.Also, a cylindrical alkaline storage battery was fabricated with thesame structure as the example of the present invention with theexception of using the electrode R as the positive electrode 12. Withthis positive electrode 12, winding was performed in a manner such thatthe surface having grooves comes to the outer periphery. This battery isreferred to as battery R.

Penetrability of liquid electrolyte was studied on the electrodes P, Q,and R. To begin with, the electrodes P and R were placed with thesurface having grooves upward, the electrode Q was placed with thesmooth pressed surface at a horizontal position, 5 ml of 40%concentration alkaline liquid electrolyte was dropped on the electrodes,and the time of absorption until the drops disappeared was measured asthe speed of penetration of the liquid electrolyte. FIG. 6 shows theresults.

As can be seen from the graph, the penetration time of the liquidelectrolyte was approximately 20 minutes for the smooth electrode Q onwhich grooves 4 had not been formed, whereas, with the electrode R onwhich grooves had been formed, the time was approximately 15 minutesindicating some improvement. Furthermore, the electrode P provided withan in-groove active material layer 7 of the present invention showed apenetration time of approximately 5 minutes.

The above results appear to suggest that, with the electrode Q in whicha dense surface layer has been formed over the entire surface, the densesurface layer hampers the penetration of the liquid electrolyte toinside. Also, the reason for some improvement over the electrode Q ofthe speed of absorption of the liquid electrolyte by the electrode R isconsidered to be due to the fact that an active material surface layerthinner than the surface layer 11 has been formed on the walls of thegrooves although a dense surface layer formed inside the grooves whenforming the grooves exists. On the other hand, the reason for greatimprovement in the speed of absorbing the liquid electrolyte of theelectrode P of the present invention is considered to be due to the factthat a dense surface layer has not been formed on the walls of thegrooves and that the active material layer 7 formed in the grooves is acoarse pressed body with a high penetrability of liquid electrolyte.

Next, improvement in windability was studied. The probability ofoccurrence of insertion failures while inserting a spiral electrodegroup into a cylindrical case was studied when 1000 pieces each ofcylindrical batteries were fabricated.

As a result, with the battery Q, as many as 50 insertion failuresoccurred. Conversely, no insertion failure occurred with the battery Pand battery R.

This indicates that windability has been improved by the formation ofthe grooves that enabled an electrode group configuration close to atrue circle. It also indicates that the electrode of the presentinvention has not lost the effect on windability of forming groovesdespite being pressed to apparent flatness.

Lastly, probability of occurrence of voltage failures due to shortcircuits was studied. Probability of voltage failure occurrence wasstudied on 1000 pieces of batteries that were free from insertionfailures. The batteries were charged by a predetermined method, leftstanding for a week, and then open circuit voltage was measured.Batteries with an open circuit voltage of 1.20 v or greater were judgedgood, and those with an open circuit voltage of below 1.20 v were judgeddefective.

FIG. 7 shows the number of occurrence of defectives. As shown by thegraph, 45 pieces of voltage failure batteries occurred with thebatteries Q using the electrode Q that does not have grooves. Also, withthe batteries R using the electrode R that has grooves but has noin-groove active material layer formed, the number of voltage failurebatteries was 25 pieces. Compared with this, with batteries P that usedthe electrode P provided with an in-groove active material layer of thepresent invention, voltage failure was zero.

Analyses of the above defective batteries have revealed that, in theelectrode Q in which no groove had been formed, cracks occurred randomlyand at irregular locations on the surface of the electrode, thus thedamaged portions piercing the separator, or active material particlesflowing out from the cracks to be sandwiched between the electrode andthe separator thus causing minute short circuits.

Also, with the conventional electrode R provided with grooves, cracksdifferent from the random cracks of the electrode Q had been formedmainly inside the grooves, and either the burrs of the cracks bulged tothe surface and projected out, or the active material flowing out fromthe cracks reached the electrode surface passing through the cavity ofthe grooves thus oozing in the separator, entered between the separatorand the electrode thus causing large and small short circuits at variouslocations.

Contrary to this, with the batteries using electrode P of the presentinvention, although occurrence of cracks was observed at the line ofintersection of the bottom 10 of the grooves and the walls, it was foundthat the area of cracks had been sealed by the in-groove active materiallayer thus protecting against flowing out of the active material orbulging or projecting of burrs. That is, it has been found that thepresent invention shows extraordinary effect in improving liquidelectrolyte penetrability of an electrode using a three-dimensionalporous body, improving flexibility of the electrode, and controllingshort circuits during winding, thus greatly improving reliability ofalkaline storage batteries suitable for high capacity using athree-dimensional porous body.

In this example, although a description has been made on a positiveelectrode, the fundamental structure and the method of fabrication of anelectrode are the same for a negative electrode, and there is noquestion that similar effect can be obtained.

INDUSTRIAL APPLICATION

As has been described above, the present invention has an effect ofimproving the speed of penetration of a liquid electrolyte intoelectrodes irrespective of the configuration of the battery and, in acylindrical alkaline storage battery, of improving windability of theelectrodes and reducing possibility of short circuits. It exhibits ahigh reliability not only in small power supplies for personalcomputers, mobile telephones, and small power appliances such as smallpower tools, lawn mowers, etc., but also in wide applications includingpower supplies for electric vehicles and hybrid electric vehicles.

What is claimed is:
 1. An electrode for an alkaline storage battery,said electrode comprising: an active material-filled substratecomprising a metal porous body substrate having three-dimensionallyconnected space and an active material filled therein; said substratehaving a plurality of grooves on one side thereof; and an in-grooveactive material layer with a low active material filling density formedin the grooves; wherein a surface of said electrode, other than wheresaid grooves are located, has a high active material filling densityformed by pressing.
 2. The electrode for an alkaline storage battery ofclaim 1, wherein a dense surface layer and a coarse surface formed onthe in-groove active material layer are alternately and regularlyrepeated in parallel to form a coarse and dense surface.
 3. Theelectrode for an alkaline storage battery of claim 1, wherein theskeleton of the metal porous body in contact with the in-groove activematerial layer does not have a fracture.
 4. A method of manufacturing anelectrode for an alkaline storage battery, the method comprising thesteps of filling an active material by forming an active material-filledsubstrate by filling active material paste into a metal porous bodysubstrate having three-dimensionally connected space, forming groovesfilled with said active material; on one side of the activematerial-filled substrate, and pressing the electrode formed with thegrooves to a predetermined thickness.
 5. The method of manufacturing anelectrode for an alkaline storage battery of claim 4, wherein the stepof filling an active material is a step of filling in a manner such thatthe active material paste does not pierce from one side of the metalporous body to the opposite side, and the step of forming grooves is astep of forming grooves on the side the active material paste is filled.6. The method of manufacturing an electrode for an alkaline storagebattery of claim 4, wherein the ratio of the depth of the grooves to thethickness of the groove-formed substrate made by forming grooves on oneside of the active material-filled substrate is in the range of 20% to50%.
 7. The method of manufacturing an electrode for an alkaline storagebattery of claim 4, wherein the configuration of cross section cutperpendicular to a groove is one comprising walls formed by the shape oftwo arcs and a flat bottom of the groove parallel to the surface.
 8. Themethod of manufacturing an electrode for an alkaline storage battery ofclaim 7, wherein an angle of invasion as defined by the angle formed bytwo tangents on a cross section perpendicular to the groove at the lineof intersection of the bottom of the groove and both walls is in therange of 45 degrees to 100 degrees.
 9. The method of manufacturing anelectrode for an alkaline storage battery of claim 4, wherein the stepof pressing the electrode formed with grooves nearly smooth is performedby a pair of smoothing roller having an axis perpendicular to thedirection of the grooves.
 10. An alkaline storage battery including anelectrode group made by winding a positive electrode and a negativeelectrode with a separator interposed, wherein at least one of thepositive and negative electrodes comprises a metal porous body havingthree-dimensionally connected space and an active material filledtherein, a surface is configured by an in-groove active material layermade by pressing a groove-formed substrate layer in which a plurality ofparallel grooves are formed on one side and a dense surface layer, andis wound around the axis of winding parallel to the direction of thegrooves with the side having the in-groove active material layer facingoutward.
 11. An alkaline storage battery of claim 10, wherein cracks arepreferentially formed in a wound electrode starting at the line ofintersection of the bottom and the walls of the plurality of groovesformed in parallel and the cracks are pressed by the in-groove activematerial layer.
 12. The electrode for an alkaline storage battery ofclaim 2, wherein the skeleton of the metal porous body in contact withthe in-groove active material layer does not have a fracture.
 13. Themethod of manufacturing an electrode for an alkaline storage battery ofclaim 5, wherein the ratio of the depth of the grooves to the thicknessof the groove-formed substrate made by forming grooves on one side ofthe active material-filled substrate is in the range of 20% to 50%. 14.The method of manufacturing an electrode for an alkaline storage batteryof claim 5, wherein the configuration of cross section cut perpendicularto a groove is one comprising walls formed by the shape of two arcs anda flat bottom of the groove parallel to the surface.
 15. The method ofmanufacturing an electrode for an alkaline storage battery of claim 6,wherein the configuration of cross section cut perpendicular to a grooveis one comprising walls formed by the shape of two arcs and a flatbottom of the groove parallel to the surface.
 16. The method ofmanufacturing an electrode for an alkaline storage battery of claim 14,wherein the configuration of cross section cut perpendicular to a grooveis one comprising walls formed by the shape of two arcs and a flatbottom of the groove parallel to the surface.
 17. An electrode accordingto claim 1, wherein said high active material filling density is higherthan said low active material filling density.